Hydrophilic, open-cell, elastic foams with a melamine/formaldehyde resin base, production thereof and use thereof in hygiene products

ABSTRACT

Hydrophilic open-celled resilient foams containing melamine-formaldehyde resins, characterized by a droplet absorption rate of less than 5 seconds and an EU standard EN ISO 14184-1 formaldehyde emission of less than 100 mg of formaldehyde/kg of foam, and obtainable by 
     (a) heating an aqueous solution or dispersion each containing at least a melamine-formaldehyde precondensate, an emulsifier, a blowing agent and a curing agent to form a foam and crosslink the precondensate, 
     (b) then conditioning the foam at from 120 to 300° C. for from 1 to 180 minutes to remove volatiles, and 
     (c) treating the foam during the conditioning or thereafter with at least one hydrophilicizer and/or with ozone, a corona discharge or a plasma, 
     are useful in hygiene articles to acquire, distribute and immobilize body fluids.

CROSS REFERENCE TO RELATED APPLICATION

This is the U.S. national phase application of International ApplicationNo. PCT/EP01/10848, filed Sep. 20, 2001.

The present invention relates to hydrophilic open-celled resilient foamsbased on melamine-formaldehyde resins, their preparation and their usein hygiene articles.

EP-A-0 017 621 and EP-A-0 017 672 disclose open-celled resilient foamsbased on melamine-formaldehyde condensation products and to processesfor their preparation. The process known from EP-A-0 037 470 producesopen-celled resilient foams from melamine-formaldehyde condensationproducts in a particularly advantageous manner by the action ofmicrowave energy (ultrahigh frequency irradiation) on an aqueoussolution or dispersion each containing a melamine-formaldehydeprecondensate, an emulsifier, a blowing agent and a curing agent. Thesolution or dispersion is heated in such a way that it foams up andcures the precondensate. The foams thus obtainable emit small amounts offormaldehyde at a rate which increases with increasing foam temperatureand moisture content.

The construction of hygiene articles and use of open-celled foams ofmelamine-formaldehyde resins as absorbent interlayer are extensivelydescribed in prior DE application no. 100 34 505.0, unpublished at thepriority date of the present invention. The melamine-formaldehyde resinfoams recited therein are highly hydrophilic, but they give off acomparatively large amount of formaldehyde on contact with body fluids.This substantially limits the possibility of using such foams in hygienearticles.

Prior DE application no. 100 27 770.5, unpublished at the priority dateof the present invention, describes the preparation of foams fromlow-formaldehyde open-celled melamine-formaldehyde resins having a molarratio of melamine to formaldehyde in the range from 1:1.0 to 1:1.9.These foams emit less than 30 mg of formaldehyde per kg of foam evenunder the warm-moist conditions customary in the hygiene sector (EUstandard EN ISO 14 184-1, water immersion at 40° C. for 1 h). Theyconsequently meet the baby clothing requirements of Oeko-Tex Standard100 (quality mark of textiles tested for harmful substances). However,the appreciable reduction in formaldehyde emission comes at the expenseof a partial loss of hydrophilic properties of the foam, as a result ofwhich the liquid acquisition rate of such foam layers decreases.

WO-A-96/21682 discloses foams which, owing to their open-celledstructure, are very useful for absorbing aqueous body fluids, especiallyblood. The foams are obtained by polymerization of (C₄-C₁₄)alkylacrylates, (C₆-C₁₆)alkyl methacrylates, (C₄-C₁₂)alkylstyrenes asmonomers, preferably styrene and ethylstyrene as comonomers, alsoaromatic polyvinyl compounds as crosslinkers; optionally polyfunctionalacrylates, methacrylates, acrylamides and methacrylamides and mixturesthereof as additional cross-linker substances. The polymerization takesplace within a High Internal Phase Emulsion (HIPE) of the W/O type inwhich the weight ratio of water phase to oil phase is in the range from20:1 to 125:1. After the polymerization has ended, the polymer foams arewashed and dried.

WO-A-97/07832, U.S. Pat. Nos. 5,318,554 and 5,550,167 concern theproduction of open-celled foams based on HIPE emulsions and their use inhygiene articles to absorb aqueous body fluids. However, the open-celledfoams are always used together with other components responsible forultimate absorption (immobilization) of the body fluids. The materialshave good application advantages, but also clear disadvantages. Forinstance, the production of these materials is an extremely complicatedprocess which is difficult to control. The enormous amount of aqueousphase (aqueous salt solution) required is neither economically norecologically sensible. Moreover, the materials are hydrophilicized atthe surface with a salt layer. This layer can become detached during useand wash into the storage medium of the absorbent core. The storagemedium is generally made of superabsorbents. It is known thatsuperabsorbents are susceptible to “salt poisoning,” i.e., theirabsorbency decreases dramatically with the increasing salt content ofthe solution to be absorbed. It is therefore certainly not desirable toadditionally increase the salt load in the body fluids to be absorbed.

It is an object of the present invention to provide open-celledresilient foams based on melamine-formaldehyde resins that arehydrophilic and whose formaldehyde emissions are substantially reducedcompared to existing foams of melamine-formaldehyde resins.

We have found that this object is achieved by hydrophilic open-celledresilient foams comprising melamine-formaldehyde resins, characterizedby a droplet absorption rate of less than 5seconds and an EU standard ENISO 14184-1 formaldehyde emission of less than 100 mg of formaldehyde/kgof foam.

Such foams have, for example, a density of from 5 to 200 g/l, a specificsurface area (determined according to BET) of more than 0.5 m²/g and aFree Swell Capacity of more than 20 g/g. They have, for example, in thewet state a tensile strength of >60 J/m².

The invention also provides a process for preparing hydrophilicopen-celled resilient foams, which comprises

(a) heating an aqueous solution or dispersion each containing at least amelamine-formaldehyde precondensate, an emulsifier, a blowing agent anda curing agent to form a foam and crosslink the precondensate,

(b) then conditioning the foam at from 120° C. to 300° C. for from 1 to180 minutes to remove volatiles, and

(c) treating the foam during the conditioning or thereafter with atleast one hydrophilicizer and/or with ozone, a corona discharge or aplasma.

Process step (a) is known from the prior art, cf. the above-discussedreferences EP-A-0 017 621, EP-A-0 017 672 and EP-A-0 037 470. The molarratio of melamine to formaldehyde is, for example, in the range from1:1.0 to 1:5 and is preferably in the range from 1:1.0 to 1:1.9. Thepreferred range is known from prior DE application no. 100 27 770.5,unpublished at the priority date of the present invention. To producelow-formaldehyde and melamine-formaldehyde resins it is particularlyadvantageous for the molar ratio of melamine to formaldehyde to bewithin the range from 1:1.3 to 1:1.8. The foaming as per step (a) iseffected by heating the mixture to a temperature above the boiling pointof the blowing agent and is carried out, for example, in such a way thatinitially there is little increase in the viscosity and a steep rise inthe viscosity and crosslinking substantially does not occur until thefoaming process has ended. However, foaming of the mixture andcrosslinking of the precondensate may also be effected concurrently.Heating of the mixture is effected, for example, using hot air or steamand/or by utilizing heat of reaction. The foaming of the aqueous mixtureof melamine-formaldehyde precondensate, emulsifier, blowing agent andcuring agent is preferably effected by means of microwaves “by theaction of microwave energy” according to the process known from EP-A-0037 470.

Structure and mechanical properties of the foams are known from EP-A-0017 672:

The DIN 53 420 density is in the range from 1.6 to 30, preferably from 2to 20 [g/l];

the DIN 52 612 coefficient of thermal conductivity is less than 0.06,preferably less than 0.04 [W.m⁻¹.K⁻¹];

the DIN 53 577 compression hardness on 60% compression, divided by thedensity, is less than 0.3, preferably less than 0.2 [N.cm⁻²/g.l⁻¹], thedetermination of the compression hardness at 60% compression having tobe followed by a recovery of the foam to at least 70%, preferably atleast 90%, especially 95%, of its original dimensions;

the modulus of elasticity on the lines of DIN 53 423, divided by thedensity, is less than 0.25, preferably less than 0.15 [N.mm⁻²/g.l⁻¹].

the DIN 53 423 bending travel on fracture is more than 10, preferablymore than 15 [mm];

the DIN 53 527 compression set on 50% compression is less than 45%,preferably less than 30%, especially less than 10%;

the DIN 18 165 dynamic stiffness for a sheet thickness of 50 mm is lessthan 20, preferably less than 10, especially less than 5 [N.cm⁻³];

under DIN 4102 they have at most normal flammability, preferably lowflammability;

tensile strength in the wet state >60 J/m²;

BET surface area of foam >0.5 m²/g.

The foams which are based on melamine-formaldehyde condensation productsand which are used according to the invention are open-pored. Under themicroscope, the foam structure is seen to contain a multiplicity ofinterconnected, three-dimensionally branched webs. Melamine-formaldehyderesin foams are sufficiently resilient only when the webs meet theconditions described in EP-A-0 017 672, i.e., the average ratio of weblength to web thickness is greater than 10:1, preferably greater than12:1, especially greater than 15:1, and web density is more than 1.10,preferably more than 1.20, especially more than 1.30 g/cm³. Web lengthand thickness is determined under the microscope, for example, and thedensity of the foam webs is determined according to Archimedes'principle by dipping the foams into a suitable liquid such asisopropanol, see EP-A-0 017 672.

In process step (b), the foam is conditioned at from 120° C. to 300° C.for from 1 to 180 minutes. It is heated to a temperature in the rangefrom 120° C. to 260° C., particularly preferably in the range from 150°C. to 250° C., for from preferably 3 minutes to 60 minutes,substantially removing water, blowing agent and formaldehyde andsupplementarily curing the foam resin. This heat treatment may becarried out immediately following foam production in the same apparatusor in a downstream apparatus; but it can also be carried out at a latertime independently of the foaming process. Conditioned foams aresubstantially less prone to shrinkage and have a lower equilibriummoisture content than products which have not been conditioned.Formaldehyde emissions are similarly substantially reduced compared tothe formaldehyde emissions of unconditioned products. Formaldehydedetachment is less than 100 mg of formaldehyde/kg of foam, preferablyless than 20 mg of formaldehyde/kg of foam (measured according to EUstandard ISO 14184-1).

The foams can be produced as sheets, blocks or webs up to 2 m in heightor as films a few mm in thickness, for example, in the range from 0.5 to7 mm. The preferred foam height (in the foam rise direction) is in therange from 10 cm to 100 cm for 2.45 GHz microwaves. All desired sheet orfleece thicknesses can be cut out of such foam blocks.

To increase the absorption rate of the foams for water and body fluids,they are hydrophilicized in process step (c). The hydrophilicization mayalso be carried out during the conditioning, for example, by having hotair flow through the melamine-formaldehyde resin foam to remove allvolatiles and adding hydrophilicizing substances, for example, in theform of an aerosol to this conditioning air. This makes it possible toobtain a hydrophilicization without the foams having to be subsequentlytreated with a hydrophilicizer.

The generation of the necessary hydrophilicity in process step (c) canbe effected in various ways, for example, by treatment with at least onehydrophilicizer and/or ozone, a corona discharge or a plasma.

When the foam is treated with a hydrophilicizer, this may take the form,for example, of an adsorption of a more hydrophilic component, forexample, of surfactants or hydrophilic polymers, which optionallyexhibit a hydrophobic modification, or a chemical attachment ofhydrophilic reagents, for example, of polyamines, polyepoxides orpolycarboxylic acids on the surface of the foam. Hydrophilicization ofthe surface of the foam may also be effected by applying crosslinkedpolymers or a crosslinked hydrophilic sheath, for example, by having

reagents capable of forming a network with themselves such ascondensation products of epichlorohydrin and polyamidoamines orpolyamines or

monomers or polymers capable of reacting with an added crosslinker, forexample, polycarboxylic acids in combination with multifunctionalepoxides, polyhydric alcohols or polyamines, polyamines in combinationwith multifunctional epoxides, acrylates or esters

act on the foams. The hydrophilicizers are normally employed indissolved form by dissolving them in a solvent. They may also be appliedin the form of aqueous dispersions or dispersions in an organic solventto the foams to be hydrophilicized. The hydrophilicization may beeffected, for example, by dipping the melamine-formaldehyde foam bodyinto the liquid which contains the hydrophilicizer in dissolved or indispersed form. Alternatively the liquid with the dissolved or dispersedhydrophilicizer may also be sprayed on the foam surface. The solvent isthereafter removed from the hydrophilicized foam body, for example, bydrying the foam.

The hydrophilicizer reacts with the melamine-formaldehyde resin foam tobe hydrophilicized and is adsorbed at the polymer surfaces. The amountof hydrophilicizer added is dimensioned in such a way that, on the onehand, a hydrophilicization is brought about without, on the other hand,disrupting the mechanical properties of the foam (flexibility).Preferably the hydrophilicizer is added in such an amount that theresulting amount of hydrophilicizer is in the range from 0.05 to 100% byweight preferably in the range from 0.1 to 50% by weight, especially inthe range from 0.2 to 30% by weight, based on the foam.

Hydrophilicization of the melamine-formaldehyde foam is possible, forexample, through the action of at least one surfactant on the foam.Particular preference is given to adding skin-friendly surfactants.Examples of skin-friendly hydrophilicizers are oil-soluble surfactantssuch as sorbitan fatty acid esters, polyglycerol fatty acid esters andpolyoxyethylene. Examples of surfactants of the above type are TRIODAN®20, a commercially available polyglycerol ester, and EMSORB® 2502, asorbitan sesquioleate. Preferred sorbitan fatty acid esters are sorbitanlaurate (e.g., SPAN® 20), sorbitan monooleate (SPAN® 80) andcombinations of sorbitan trioleate (SPAN® 85) and sorbitan monooleate(SPAN® 80). Particular preference is given to the combination ofsorbitan monooleate and sorbitan trioleate in a weight ratio of not lessthan 3:1, particularly preferably of 4:1. Combinations of sorbitanlaurate with certain polyglycerol fatty acid esters are also used ashydrophilicizers. Polyglycerol fatty acid esters are obtained fromester-forming polyglycerols and fatty acids, cf., for example, U.S. Pat.No. 3,637,774. Polyglycerols are characterized by a high fraction oflinear (especially acyclic) diglycerols, a low fraction of tri- orhigher polyglycerols and a low fraction of cyclic diglycerols. Theweight ratio of sorbitan laurate to polyglycerol fatty acid ester isnormally in the range from 10:1 to 1:10, preferably in the range from4:1 to 1:1.

Preference is further given to organomodified polydimethylsiloxanes ofthe type NUWET® 500 or modified silicones of the type NUWET® 300 (fromOSi) Both the polydimethylsiloxanes and the polysilicones are modifiedto be hydrophilic. This modification may be effected through theincorporation of amino, carboxyl or hydroxyl groups. It is likewisepossible to attach oligo- or polyethylene glycol side chains.

Useful hydrophilicizers further include acylated polyamines, obtainable,for example, by reaction of polyamines with monobasic carboxylic acids.Useful polyamines include, for example, polyalkylenepolyamines havingaverage molar masses of from 300 to 1 million, preferably of from 500 to500,000. Preferred polyalkylenepolyamines are polyethyleneimines.

The monobasic carboxylic acids usually have from 1 to 18 carbon atoms,for example, formic acid, acetic acid, propionic acid, lauric acid,palmitic acid or stearic acid. In some cases it is advantageous to reactmixtures of a long-chain monocarboxylic acid in succession or togetherwith a polyalkylenepolyamine. Instead of carboxylic acids it is alsopossible to use the esters of carboxylic acids. When thepolyalkylenepolyamines are reacted with the carboxylic acids or esters,the NH₂ or NH groups of the polyalkylenepolyamines are amidated. This isa way of acylating, for example, from 5 to 100%, preferably from 15 to85%, of the nitrogen atoms in the polyalkylenepolyamine.

Useful hydrophilicizers further include polymers containing

i) at least one polyisocyanate and

ii) at least one compound having at least two isocyanate-reactive groupsand additionally at least one tertiary amino group in built-in form. Atleast some of the tertiary amino groups of component ii) in the polymerare in the form of ammonium groups. Charged cationic groups can beproduced from the tertiary amine nitrogens of the compounds of componentii) and/or of the polymer either by protonation or by quaternization.Then at least a portion of the tertiary amino groups in the polymer willbe present in the form of its reaction products with at least oneneutralizing (protonating) and/or quaternizing agent.

The polyisocyanates i) are preferably selected from compounds havingfrom 2 to 5 isocyanate groups, isocyanate prepolymers having an averagenumber of from 2 to 5 isocyanate groups and mixtures thereof. It is alsopossible to use compounds which in addition to or instead of freeisocyanate groups have functional groups which release isocyanate groupsor react like isocyanate groups. These include, for example, compoundshaving blocked isocyanate groups, uretdione groups, isocyanurate groupsand/or biuret groups. The compounds having isocyanurate groups are inparticular simple triisocyanatoisocyanurates, i.e., cyclic trimers ofdiisocyanates, or mixtures with their higher homologs having more thanone isocyanurate ring. Useful compounds of component ii) furtherinclude, for example, tertiary amines where the amine nitrogen has threesubstituents, which are preferably hydroxyalkyl and/or aminoalkylgroups. Preferred compounds used for component ii) are, for example,bis(aminopropyl)methylamine, bis(aminopropyl)piperazine,methyldiethanolamine and mixtures thereof.

Useful cationic polymers include all cationic synthetic polymerscontaining amino and/or ammonium groups. Examples of such cationicpolymers are vinylamine polymers, vinylimidazole polymers, polymerscontaining quaternary vinylimidazol units, condensates of imidazole andepichlorohydrin, crosslinked polyamidoamines, ethyleneimine-graftedcrosslinked polyamidoamines, polyethyleneimines, alkoxylatedpolyethyleneimines, crosslinked polyethyleneimines, amidatedpolyethyleneimines, alkylated polyethyleneimines, polyamines,amine-epichlorohydrin polycondensates, water-soluble polyadditionproducts of multifunctional amines with multifunctional epoxides,alkoxylated polyamines, polyallylamines, polydimethyldiallylammoniumchlorides, polymers containing basic (meth)acrylamide or (meth)acrylicester units, polymers containing basic quaternary (meth)acrylamide or(meth)acrylic ester units, and/or lysine condensates.

Cationic polymers also include amphoteric polymers having a net cationiccharge, i.e., the polymers contain not only anionic but also cationicmonomers in polymerized form, but the molar fraction of cationic unitscontained in the polymer is larger than that of the anionic units.

Vinylamine polymers (i.e., polymers containing vinylamine units) arepreparable, for example, from open-chain N-vinylcarboxamides of theformula

where R¹ and R² are identical or different and are each hydrogen or C₁-to C₆-alkyl. Useful monomers include, for example, N-vinylformamide(R¹═R²═H in formula I), N-vinyl-N-methylformamide, N-vinylacetamide,N-vinyl-N-methylacetamide, N-vinyl-N-ethylacetamide,N-vinyl-N-methylpropionamide and N-vinylpropionamide. To prepare thepolymers, the monomers mentioned may be polymerized alone, mixed witheach other or together with other monoethylenically unsaturatedmonomers. Homo- and copolymers of N-vinylformamide are preferred asstarting materials. Vinylamine polymers are known, for example, fromU.S. Pat. No. 4,421,602, U.S. Pat. No. 5,334,287, EP-A-0 216 387 andEP-A-0 251 182. They are obtained by acid, base or enzymatic hydrolysisof polymers containing units derived from monomers of the formula I.

Useful monoethylenically unsaturated monomers for copolymerization withN-vinylcarboxamides include all compounds copolymerizable therewith.Examples thereof are vinyl esters of saturated carboxylic acids of from1 to 6 carbon atoms such as vinyl formate, vinyl acetate, vinylpropionate and vinyl butyrate and vinyl ethers such as C₁- to C₆-alkylvinyl ethers, e.g., methyl or ethyl vinyl ether. Useful comonomersfurther include ethylenically unsaturated C₃- to C₆-carboxylic acids,for example, acrylic acid, methacrylic acid, maleic acid, crotonic acid,itaconic acid and vinylacetic acid and also their alkali metal andalkaline earth metal salts, esters, amides and nitrites of thecarboxylic acids mentioned, for example, methyl acrylate, methylmethacrylate, ethyl acrylate and ethyl methacrylate.

Further useful carboxylic esters are derived from glycols orpolyalkylene glycols where in each case only one OH group is esterified,for example, hydroxyethyl acrylate, hydroxyethyl methacrylate,hydroxypropyl acrylate, hydroxybutyl acrylate, hydroxypropylmethacrylate, hydroxybutyl methacrylate and also monoacrylate esters ofpolyalkylene glycols having a molar mass of from 500 to 10,000. Usefulcomonomers further include esters of ethylenically unsaturatedcarboxylic acids with aminoalcohols, for example, dimethylaminoethylacrylate, dimethylaminoethyl methacrylate, diethylaminoethyl acrylate,diethylaminoethyl methacrylate, dimethylaminopropyl acrylate,dimethylaminopropyl methacrylate, diethylaminopropyl acrylate,dimethylaminobutyl acrylate and diethylaminobutyl acrylate. Basicacrylates can be used in the form of the free bases, the salts withmineral acids such as hydrochloric acid, sulfuric acid or nitric acid,the salts with organic acids such as formic acid, acetic acid, propionicacid or sulfonic acids or in quaternized form. Useful quaternizingagents include, for example, dimethyl sulfate, diethyl sulfate, methylchloride, ethyl chloride or benzyl chloride.

Useful comonomers further include amides of ethylenically unsaturatedcarboxylic acids such as acrylamide, methacrylamide and alsoN-alkylmonoamides and diamides of monoethylenically unsaturatedcarboxylic acids with alkyl radicals of from 1 to 6 carbon atoms, forexample, N-methylacrylamide, N,N-dimethylacrylamide,N-methylmethacrylamide, N-ethylacrylamide and N-propylacrylamide andtert-butylacrylamide and also basic (meth)acrylamides, for example,dimethylaminoethylacrylamide, dimethylaminoethylmethacrylamide,diethylaminoethylacrylamide, diethylaminoethylmethacrylamide,dimethylaminopropylacrylamide, diethylaminopropylacrylamide,dimethylaminopropylmethacrylamide and diethylaminopropylmethacrylamide.

Useful comonomers further include N-vinylpyrrolidone,N-vinylcaprolactam, acrylonitrile, methacrylonitrile, N-vinylimidazoleand also substituted N-vinylimidazoles, for example,N-vinyl-2-methylimidazole, N-vinyl-4-methylimidazole,N-vinyl-5-methylimidazole, N-vinyl-2-ethylimidazole andN-vinylimidazolines such as N-vinylimidazoline,N-vinyl-2-methylimidazoline and N-vinyl-2-ethylimidazoline.N-Vinylimidazoles and N-vinylimidazolines are used not only in the formof the free bases but also after neutralization with mineral acids ororganic acids or after quaternization, a quaternization being preferablyeffected with dimethyl sulfate, diethyl sulfate, methyl chloride orbenzyl chloride. Also useful are diallyldialkylammonium halides, forexample, diallyldimethylammonium chlorides.

Useful comonomers further include sulfo-containing monomers, forexample, vinylsulfonic acid, allylsulfonic acid, methallylsulfonic acid,styrenesulfonic acid, the alkali metal or ammonium salts of these acidsor 3-sulfopropyl acrylate. Since the amphoteric copolymers contain morecationic units than anionic units, they have a cationic charge overall.

The copolymers contain, for example

from 99.99 to 1 mol %, preferably from 99.9 to 5 mol %, ofN-vinylcarboxamides of the formula I and

from 0.01 to 99 mol %, preferably from 0.1 to 95 mol %, of othermonoethylenically unsaturated monomers copolymerizable therewith

in copolymerized form.

To prepare vinylamine polymers it is preferable to start fromhomopolymers of N-vinylformamide or from copolymers obtainable bycopolymerization of

N-vinylformamide with

vinyl formate, vinyl acetate, vinyl propionate, acrylonitrile,N-vinylcaprolactam, N-vinylurea, acrylic acid, N-vinylpyrrolidone or C₁-to C₆-alkyl vinyl ethers

and subsequent hydrolysis of the homo- or copolymers to form vinylamineunits from the copolymerized N-vinylformamide units, the degree ofhydrolysis being, for example, in the range from 0.1 to 100 mol %.

The hydrolysis of the above-described polymers is effected according toknown processes by the action of acids, bases or enzymes. This convertsthe copolymerized monomers of the above-indicated formula I throughdetachment of the group

where R² is as defined for formula I, into polymers which containvinylamine units of the formula

 where R¹ is as defined for formula I. When acids are used ashydrolyzing agents, the units III are present as ammonium salt.

The homopolymers of the N-vinylcarboxamides of the formula I and theircopolymers may be hydrolyzed to an extent in the range from 0.1 to 100mol %, preferably to an extent in the range from 70 to 100 mol %. Inmost cases, the degree of hydrolysis of the homo- and copolymers is inthe range from 5 to 95 mol %. The degree of hydrolysis of thehomopolymers is synonymous with the vinylamine units content of thepolymers. In the case of copolymers containing units derived from vinylesters, the hydrolysis of the N-vinylformamide units can be accompaniedby a hydrolysis of the ester groups with the formation of vinyl alcoholunits. This is the case especially when the hydrolysis of the copolymersis carried out in the presence of aqueous sodium hydroxide solution.Copolymerized acrylonitrile is likewise chemically modified in thehydrolysis, for example, converted into amide groups or carboxyl groups.The homo- and copolymers containing vinylamine units may optionallycontain up to 20 mol % of amidine units, formed, for example, byreaction of formic acid with two adjacent amino groups or byintramolecular reaction of an amino group with an adjacent amide group,for example, of copolymerized N-vinylformamide. The molar masses ofvinylamine polymers range, for example, from 1,000 to 10 million,preferably from 10,000 to 5 million (determined by light scattering).This molar mass range corresponds, for example, to K values of from 5 to300, preferably from 10 to 250 (determined by the method of H.Fikentscher in 5% aqueous sodium chloride solution at 25° C. and apolymer concentration of 0.5% by weight).

The vinylamine polymers are preferably used in salt-free form. Salt-freeaqueous solutions of vinylamine polymers are preparable, for example,from the above-described salt-containing polymer solutions by means ofultrafiltration using suitable membranes having molecular weight cutoffsat, for example, from 1,000 to 500,000 daltons, preferably from 10,000to 300,000 daltons. The hereinbelow described aqueous solutions of otherpolymers containing amino and/or ammonium groups are likewise obtainablein salt-free form by means of ultrafiltration.

Polyethyleneimines are prepared, for example, by polymerizingethyleneimine in an aqueous solution in the presence of acid-detachingcompounds, acids or Lewis acids as catalyst. Polyethyleneimines have,for example, molar masses of up to 2 million, preferably of from 200 to500,000. Particular preference is given to using polyethyleneimineshaving molar masses of from 500 to 100,000. Useful polyethyleneiminesfurther include water-soluble crosslinked polyethyleneimines which areobtainable by reaction of polyethyleneimines with crosslinkers such asepichlorohydrin or bischlorohydrin ethers of polyalkylene glycolscontaining from 2 to 100 ethylene oxide and/or propylene oxide units.Also useful are amidic polyethyleneimines which are obtainable, forexample, by amidation of polyethyleneimines with C₁- toC₂₂-monocarboxylic acids. Useful cationic polymers further includealkylated polyethyleneimines and alkoxylated polyethyleneimines.Alkoxylation is carried out using, for example, from 1 to 5 ethyleneoxide or propylene oxide units per NH unit in the polyethyleneimine.

Useful polymers containing amino and/or ammonium groups also includepolyamidoamines, which are preparable, for example, by condensingdicarboxylic acids with polyamines. Useful polyamidoamines are obtained,for example, when dicarboxylic acids having from 4 to 10 carbon atomsare reacted with polyalkylenepolyamines containing from 3 to 10 basicnitrogen atoms in the molecule. Useful dicarboxylic acids include, forexample, succinic acid, maleic acid, adipic acid, glutaric acid, subericacid, sebacic acid or terephthalic acid. Polyamidoamines may also beprepared using mixtures of dicarboxylic acids as well as mixtures ofplural polyalkylenepolyamines. Useful polyalkylenepolyamines include,for example, diethylenetriamine, triethylenetetramine,tetraethylenepentamine, dipropylenetriamine, tripropylenetetramine,dihexamethylenetriamine, aminopropylethylenediamine andbisaminopropylethylenediamine. The dicarboxylic acids andpolyalkylenepolyamines are heated at an elevated temperature, forexample, at from 120° C. to 220° C., preferably at from 130° C. to 180°C., to prepare polyamidoamines. The water of condensation formed isremoved from the system. The condensation may also employ lactones orlactams of carboxylic acids having from 4 to 8 carbon atoms. The amountof a polyalkylenepolyamine used per mole of a dicarboxylic acid is, forexample, in the range from 0.8 to 1.4 mol.

Amino-containing polymers further include ethyleneimine-graftedpolyamidoamines. They are obtainable from the above-describedpolyamidoamines by a reaction with ethyleneimine in the presence ofacids or Lewis acids such as sulfuric acid or boron trifluorideetherates at, for example, from 80° C. to 100° C. Compounds of this kindare described, for example, in DE-B-24 34 816.

Useful cationic polymers also include crosslinked or uncrosslinkedpolyamidoamines which may additionally have been grafted withethyleneimine prior to crosslinking. Crosslinked ethyleneimine-graftedpolyamidoamines are water-soluble and have, for example, an averagemolar weight of from 3,000 to 1 million daltons. Customary crosslinkersinclude, for example, epichlorohydrin or bischlorohydrin ethers ofalkylene glycols and polyalkylene glycols.

Further examples of cationic polymers that contain amino and/or ammoniumgroups are polydiallyldimethylammonium chlorides. Polymers of this kindare likewise known.

Useful cationic polymers further include copolymers of, for example,1-99 mol %, preferably 30-70 mol %, of acrylamide and/or methacrylamideand 99-1 mol %, preferably 70-30 mol %, of cationic monomers such asdialkylaminoalkylacrylamide, dialkylaminoalkyl acrylate,dialkylaminoalkylmethacrylamide and/or dialkylaminoalkyl methacrylate.Basic acrylamides and methacrylamides are preferably likewise present inacid-neutralized form or in quaternized form. Examples areN-trimethylammoniumethylacrylamide chloride,N-trimethylammoniumethylmethacrylamide chloride,N-trimethylammoniumethyl methacrylate chloride, N-trimethylammoniumethylacrylate chloride, trimethylammoniumethylacrylamide methosulfate,trimethylammoniumethylmethacrylamide methosulfate,N-ethyldimethylammoniumethylacrylamide ethosulfate,N-ethyldimethylammoniumethylmethacrylamide ethosulfate,trimethylammoniumpropylacrylamide chloride,trimethylammoniumpropylmethacrylamide chloride,trimethylammoniumpropylacrylamide methosulfate,trimethylammoniumpropylmethacrylamide methosulfate andN-ethyldimethylammoniumpropylacrylamide ethosulfate. Preference is givento trimethylammoniumpropylmethacrylamide chloride.

Further useful cationic monomers for preparing (meth)acrylamide polymersare diallyldimethylammonium halides and also basic (meth)acrylates.Useful examples are copolymers of 1-99 mol %, preferably 30-70 mol %, ofacrylamide and/or methacrylamide and 99-1 mol %, preferably 70-30 mol %,of dialkylaminoalkyl acrylates and/or methacrylates such as copolymersof acrylamide and N,N-dimethylaminoethyl acrylate or copolymers ofacrylamide and dimethylaminopropyl acrylate. Basic acrylates ormethacrylates are preferably present in acid-neutralized form orquaternized form. Quaternization may be effected, for example, withmethyl chloride or with dimethyl sulfate.

Useful polymers having amino and/or ammonium groups also includepolyallylamines. Polymers of this kind are obtained byhomopolymerization of allylamine, preferably in acid-neutralized form orin quaternized form, or by co-polymerizing allylamine with othermonoethylenically unsaturated monomers described above as comonomers forN-vinylcarboxamides.

The cationic polymers have, for example, K values of from 8 to 300,preferably from 15 to 180 (determined by the method of H. Fikentscher in5% aqueous sodium chloride solution at 25° C. and a polymerconcentration of 0.5% by weight). At pH 4.5, for example, they have acharge density of at least 1, preferably at least 4, meq/g ofpolyelectrolyte.

Preferred cationic polymers are polydimethyldiallylammonium chloride,polyethyleneimine, polymers containing vinylamine units,(meth)acrylmide/basic monomer copolymers, polymers containing lysineunits or mixtures thereof. Examples of preferred cationic polymers are:

polylysines of M_(w) 250-250,000, preferably 500-100,000, and alsolysine condensates having M_(w) molar masses of from 250 to 250,000, thecocondensable component being selected, for example, from amines,polyamines, ketene dimers, lactams, alcohols, alkoxylated amines,alkoxylated alcohols and/or nonproteinogenic amino acids,

vinylamine homopolymers, 1-99% hydrolyzed polyvinylformamides,copolymers of vinylformamide and vinyl acetate, vinyl alcohol,vinylpyrrolidone or acrylamide, each having molar masses of3,000-500,000,

vinylimidazole homopolymers, vinylimiazole copolymers withvinylpyrrolidone, vinylformamide, acrylamide or vinyl acetate havingmolar masses of from 5,000 to 500,000 and also quaternary derivativesthereof,

polyethyleneimines, crosslinked polyethyleneimines or amidatedpolyethyleneimines having molar masses of from 500 to 3,000,000,

amine-epichlorohydrin polycondensates which contain imidazole,piperazine, C₁-C₈-alkylamines, C₁-C₈-dialkylamines and/ordimethylaminopropylamine as amine component and have a molar mass offrom 500 to 250,000, and polymers containing basic (meth)acrylamide or(meth)acrylate ester units, polymers containing basic quaternary(meth)acrylamide or (meth)acrylate ester units having molar masses offrom 10,000 to 2,000,000.

Amino-containing polymers which have been applied as hydrophilicizers tothe melamine-formaldehyde resin foams may optionally be crosslinkedthereon. Crosslinking of the foams treated with polymers containingamino groups is obtained, for example, by reaction with at leastbifunctional crosslinkers such as epichlorohydrin, bischlorohydrinethers of polyalkylene glycols, polyepoxides, multifunctional esters,multifunctional acids or multifunctional acrylates.

The inventive foams based on melamine-formaldehyde resins are used inhygiene articles to acquire, distribute and immobilize body fluids,especially blood. Their hydrophilic character permits spontaneousacquisition of aqueous body fluids. The open-celled structure ensuresrapid transportation into the foam interior. Hygiene articles whichinclude the foams to be used according to the invention are essentiallyinfant diapers, incontinence products, femcare articles, wound contactmaterials or secondary wound dressings.

The melamine-formaldehyde resin foams for inventive use in the hygienesector are open-pored and hydrophilic. The droplet absorption rate ofthe melamine-formaldehyde foams according to the invention is less than5 seconds, preferably less than 2 seconds, particularly preferably lessthan 1 second.

The open-celled resilient foams are preferably incorporated assheet-like structures in the form of foam fleeces from 0.1 to 10 mm,preferably from 1 to 5 mm, in thickness into hygiene products such asinfant diapers, incontinence and femcare articles or as wound contactmaterials or in dressing materials. Foam density is, for example, in therange from 5 to 200 g/l, preferably from 10 to 50 g/l. The foamspreferably have a webbed structure, a BET specific surface area of morethan 0.5 m²/g, for example, in the range from 1 to 7 m²/g, a Free SwellCapacity of more than 20 g/g, for example, from 80 to 120 g/g, and atensile strength of >60 J/m², for example, from 100 to 600 J/m², in thewet state.

A hygiene article generally constitutes a combination of aliquid-impervious backsheet, a liquid-pervious topsheet, and anabsorbent interlayer core. Hygiene articles of this type are known anddescribed, for example, in DE-U-92 18 991 and EP-A-0 689 818. Theabsorbent composition is fixed between topsheet and backsheet. Elasticcuffs and self-adhesive tabs may optionally be integrated in the hygienearticle. A preferred hygiene article construction is known, for example,from U.S. Pat. No. 3,860,003.

When the hydrophilic open-celled resilient foams are used in a hygienearticle, there are, for example, two ways of configuring the absorbentinterlayer core:

1. The melamine-formaldehyde foam layer is used as the absorbentinterlayer core without further layers. It then acts simultaneously asacquisition or acquisition/distribution layer and as storage layer.

2. The absorbent interlayer core consists of (a) a melamine-formaldehydefoam layer, which acts as acquisition or acquisition/distribution layer,and (b) a storage layer containing 10-100% by weight of highly swellablehydrogel.

The storage layer either is a hydrogen layer or constitutes compositionswhich include highly swellable hydrogels or have them fixed to them. Anycomposition is suitable that is capable of accommodating highlyswellable hydrogels and being integrated into the absorbent core. Amultiplicity of such compositions is already known and described indetail in the literature. A composition for installing the highlyswellable hydrogels can be, for example, a fiber matrix consisting of acellulose fiber mixture (airlaid web, wet laid web) or of syntheticpolymer fibers (meltblown web, spun-bonded web), or else of a fiberblend of the cellulose fibers and synthetic fibers. Furthermore,open-pored foams or the like may be used to install highly swellablehydrogels.

Alternatively, such a composition can be the result of fusing twoindividual layers to form one or, better, a multiplicity of chamberswhich contain the highly swellable hydrogels. In this case, at least oneof the two layers should be water-pervious. The second layer may beeither water-pervious or water-impervious. The layer material used maybe tissues or other fabrics, closed or open-celled foams, perforatedfilms, elastomers or fabrics composed of fiber material. When thestorage layer consists of a composition of layers, the layer materialshould have a pore structure whose pore dimensions are small enough toretain the highly swellable hydrogel particles. The above examples onthe composition of the storage layer also include laminates composed ofat least two layers between which the highly swellable hydrogels can beinstalled and fixed.

Furthermore, the storage layer can consist of a carrier material, forexample, a polymer film. on which the highly swellable hydrogelparticles are fixed. The fixing can be effected not only on one side butalso on both sides. The carrier material can be water-pervious orwater-impervious.

In the above compositions of the storage layer, the highly swellablehydrogels can have a weight fraction of from 10 to 100% by weight,preferably from 40 to 100% by weight and particularly preferably from 70to 100% by weight. When the above storage layer composition constitutesa fiber matrix, then the absorbent composition results from a mixture offiber materials and highly swellable hydrogels.

The storage layer may contain manifold fiber materials, which are usedas fiber network or matrices. The present invention encompasses not onlyfibers of natural origin (modified or unmodified) but also syntheticfibers.

Examples of cellulose fibers include cellulose fibers which arecustomarily used in absorption products, such as fluff pulp and pulp ofthe cotton type. The materials (hard- or softwoods), productionprocesses, such as chemical pulp, semi-chemical pulp,chemothermomechanical pulp (CTMP) and bleaching processes, are notparticularly restricted. For example, natural cellulose fibers such ascotton, flax, silk, wool, jute, ethylcellulose and cellulose acetate areused.

Suitable synthetic fibers are produced from polyvinyl chloride,polyvinyl fluoride, polytetrafluoroethylene, polyvinylidene chloride,polyacrylic compounds such as ORLON®, polyvinyl acetate, polyethyl vinylacetate, soluble or insoluble polyvinyl alcohol. Examples of syntheticfibers include thermoplastic polyolefin fibers, such as polyethylenefibers (PULPEX®), polypropylene fibers and poly-ethylene-polypropylenebicomponent fibers, polyester fibers, such as polyethylene terephthalatefibers (DACRON® or KODEL®), copolyesters, polyvinyl acetate, polyethylvinyl acetate, polyvinyl chloride, polyvinylidene chloride,polyacrylics, polyamides, copolyamides, polystyrene and copolymers ofthe aforementioned polymers and also bicomponent fibers composed ofpolyethylene terephthalate-polyethyleneisophthalate copolymer, polyethylvinyl acetate/polypropylene, polyethylene/polyester,polypropylene/polyester, copolyester/polyester, polyamide fibers(nylon), polyurethane fibers, polystyrene fibers and polyacrylonitrilefibers. Preference is given to polyolefin fibers, polyester fibers andtheir bicomponent fibers. Preference is further given to thermallyadhesive bicomponent fibers composed of polyolefin of the core-sheathtype and side-by-side type on account of their excellent dimensionalstability following fluid absorption.

The synthetic fibers mentioned are preferably used in combination withthermoplastic fibers. In the course of the heat treatment, the lattermigrate to some extent into the matrix of the fiber material present andso constitute bond sites and renewed stiffening elements on cooling.Additionally the addition of thermoplastic fibers means that there is anincrease in the present pore dimensions after the heat treatment hastaken place. This makes it possible, by continuous addition ofthermoplastic fibers during the formation of the absorbent core, tocontinuously increase the fraction of thermoplastic fibers in thedirection of the topsheet, which results in a similarly continuousincrease in the pore sizes. Thermoplastic fibers can be formed from amultiplicity of thermoplastic polymers which have a melting point ofless than 190° C., preferably in the range from 75° C. to 175° C. Thesetemperatures are too low for damage to the cellulose fibers to belikely.

The above-described synthetic fibers may, for example, be from 1 to 200mm in length and from 0.1 to 100 denier (gram per 9,000 meters) indiameter. Preferred thermoplastic fibers are from 3 to 50 mm in length,particularly preferred thermoplastic fibers are from 6 to 12 mm inlength. The preferred diameter for the thermoplastic fiber is in therange from 1.4 to 10 decitex, and the range from 1.7 to 3.3 decitex(gram per 10,000 meters) is particularly preferred. The form of thefiber may vary; examples include woven types, narrow cylindrical types,cut/chopped yarn types, staple fiber types and continuous filament fibertypes.

The fibers in the absorbent composition of the. invention can behydrophilic and/or hydrophobic. According to the definition of Robert F.Gould in the 1964 American Chemical Society publication “Contact angle,wettability and adhesion,” a fiber is referred to as hydrophilic whenthe contact angle between the liquid and the fiber (or the fibersurface) is less than 90° or when the liquid tends to spreadspontaneously on the same surface. The two processes are generallycoexistent Conversely, a fiber is termed hydrophobic when a contactangle of greater than 90° is formed and no spreading is observed.

Preference is given to using hydrophilic fiber material. Particularpreference is given to using fiber material which is weakly hydrophilicon the body side and most hydrophilic in the region surrounding thehighly swellable hydrogels. In the manufacturing process, layers havingdifferent hydrophilicities are used to create a gradient which channelsimpinging fluid to the hydrogel, where it is ultimately absorbed.

Suitable hydrophilic fibers for use in the absorbent core of theinvention include, for example, cellulose fibers, modified cellulosefibers, rayon, polyester fibers, for example, polyethylene terephthalate(DACRON®), and hydrophilic nylon (HYDROFIL®). Suitable hydrophilicfibers may also be obtained by hydrophilicizing hydrophobic fibers, forexample, the treatment of thermoplastic fibers obtained from polyolefins(e.g., polyethylene or polypropylene, polyamides, polystyrenes,polyurethanes, etc.) with surfactants or silica. However, for costreasons and ease of availability, cellulosic fibers are preferred.

The highly swellable hydrogel particles are embedded into the fibermaterial described. This can be done in various ways, for example, byusing the hydrogel material and the fibers together to create anabsorbent layer in the form of a matrix, or by incorporating highlyswellable hydrogels into fiber mixture layers, where they are ultimatelyfixed, whether by means of adhesive or lamination of the layers.

The fluid-acquiring and distributing fiber matrix may comprise syntheticfiber or cellulosic fiber or a mixture of synthetic fiber and cellulosicfiber, in which case the mixing ratio may vary from (100 to 0) syntheticfiber:(0 to 100) cellulosic fiber. The cellulosic fibers used mayadditionally have been chemically stiffened to increase the dimensionalstability of the hygiene article.

The chemical stiffening of cellulosic fibers may be provided indifferent ways. A first way of providing fiber stiffening is by addingsuitable coatings to the fiber material. Such additives include, forexample, polyamide-epichlorohydrin coatings (KYMENE® 557 H),polyacrylamide coatings (described in U.S. Pat. No. 3, 556,932 or as thePAREZ® 631 NC commercial product), melamine-formaldehyde coatings andpolyethyleneimine coatings.

Cellulosic fibers may also be chemically stiffened by chemical reaction.For instance, suitable crosslinker substances may be added to effectcrosslinking taking place within the fiber. Suitable crosslinkersubstances are typical substances used for crosslinking monomersincluding but not limited to C₂-C₈-dialdehydes, C₂-C₈-monoaldehydeshaving acid functionality and in particular C₂-C₉polycarboxylic acids.Specific substances from this series are, for example, glutaraldehyde,glyoxal, glyoxylic acid, formaldehyde and citric acid. These substancesreact with at least 2 hydroxyl groups within any one cellulose chain orbetween two adjacent cellulose chains within any one cellulose fiber.The crosslinking causes a stiffening of the fibers, to which greaterdimensional stability is imparted as a result of this treatment. Inaddition to their hydrophilic character, these fibers exhibit uniformcombinations of stiffening and elasticity. This physical property makesit possible to retain the capillary structure even under simultaneouscontact with fluid and compressive forces and to prevent prematurecollapse.

Chemically crosslinked cellulose fibers are known, cf., for example,WO-A-91/11162. The chemical crosslinking imparts stiffening to the fibermaterial, which is ultimately reflected in improved dimensionalstability for the hygiene article as a whole. The individual layers arejoined together by methods known to one skilled in the art, for example,by intermelting by heat treatment, addition of hot-melt adhesives, latexbinders, etc.

Generally, the invention utilizes a hydrophilicized fleece of anopen-celled resilient melamine-formaldehyde resin foam having very lowformaldehyde emissions as or in the absorbent interlayer core. Thedimension (thickness) of the absorbent interlayer when used as anabsorbent core is generally in the range from 0.5 to 10 mm, preferablyin the range from 1 to 5 mm. When used as an acquisition anddistribution layer in combination with a storage layer, the thickness isin the range from 0.1 to 10 mm, preferably in the range from 0.5 to 3mm.

The topsheet can be produced in various ways, for example, as a woven,nonwoven, spun or combed fiber mixture. Preference is given to using acombed fiber mixture which is thermally bonded to form the topsheet. Thebasis weight of the topsheet is preferably in the range from 18 to 25g/m². It has a tensile strength of at least 400 g/cm in the dry stateand 55 g/cm in the wet state.

The backsheet is usually a liquid-impervious material, for example,polyolefin (a polyethylene backsheet, for example) to protect the user'sclothing from possible leakage.

The individual layers from which the hygiene articles are constructedare joined together by known methods, for example, by intermelting thelayers by heat treatment, addition of hot-melt adhesives, latex binders,etc. The absorbent interlayer core is positioned between topsheet andbacksheet.

Methods of Measurement

Droplet Absorption Rate

A single droplet of a 0.9% sodium chloride solution is pipetted onto afoam layer about 5 mm in thickness and the time taken for the droplet todisappear into the foam is recorded. The foam was rated hydrophilic whenthe absorption time was <5 sec.

Density

Any suitable gravimetric method can be used for determining the densityof the foam. What is determined is the mass of solid foam per unitvolume of foam structure. A method for density determination of the foamis described in ASTM Method No. D 3574-86, Test A. This method wasoriginally developed for the density determination of urethane foams,but can also be used for this purpose. By this method, the dry mass andvolume of a preconditioned sample is determined at 22° C.±2° C. Volumedeterminations of larger sample dimensions are carried out underatmospheric pressure.

Free Swell Capacity (FSC)

This method is used to determine the free swellability of theopen-celled resilient melamine-formaldehyde foam. To determine FSC, atestpiece of suitable size, for example, with an area of approximately 1cm×1 cm, is cut out of a foam blank and weighed. The testpiece is placedin an excess of 0.9% NaCl solution (at least 0.83 l of sodium chloridesolution/1 g of foam) for 30 minutes. The testpiece is subsequentlyallowed to drip for 10 minutes before it is hung up by one corner,avoiding compression at all costs. The amount of liquid absorbed isdetermined by weighing the testpiece.

Acquisition Time

The open-celled resilient melamine-formaldehyde foam is cut into layers1.5 or 2 or 4 mm in thickness. A commercially available diaper iscarefully cut open, the highloft used as acquisition medium removed andinstead the open-celled resilient melamine-formaldehyde foam layerinserted. The diaper is resealed. Synthetic urine solution is applied toit through a plastic plate having a ring in the middle (inner diameterof the ring 6.0 cm, height 4.0 cm). The plate is loaded with additionalweights so that the total load on the diaper is 13.6 g/cm². The plasticplate is placed on the diaper in such a way that the center of thediaper is also the center of the application ring. 60 ml of 0.9% byweight sodium chloride solution are applied three times. The sodiumchloride solution is measured in a graduated cylinder and applied to thediaper in a continuous stream through the ring in the plate. At the sametime, the time taken for the solution to penetrate completely into thediaper is recorded. The time measured is noted as acquisition time 1.Thereafter, the diaper is loaded with a plate for 20 min, the load beingmaintained at 13.6 g/cm². This is followed by the second application ofthe liquid. The time measured is noted as acquisition time 2. The samemethod is employed to determine acquisition time 3.

Specific Surface Area

Specific surface area is determined by the BET method as set forth inDIN 66132.

Formaldehyde Emission

Determined by Edana method 210.1-99 (testing to EU standard EN ISO14184-1)

One g of the foam sample to be tested is cut into small pieces,introduced into an Erlenmeyer flask together with 100 ml of water andtightly sealed. The Erlenmeyer flask is placed into a water bathmaintained at 40° C. and is left therein for 60 min with periodicshaking. Subsequently, the solution obtained is filtered off or the foamis expressed.

The formaldehyde content of the solution obtained is determined by theacetylacetone method.

Unless the context suggests otherwise, the percentages in the examplesare by weight.

EXAMPLES Comparative Example 1

Seventy-five parts of a spray-dried melamine-formaldehyde precondensate(molar ratio 1:3) were dissolved in 25 parts of water. This resinsolution was admixed with 3% of formic acid, 2% of a sodiumC₁₂/C₁₈-alkanesulfonate and 19% of pentane, each based on the resin. Themixture was vigorously stirred and subsequently foamed in apolypropylene foaming mold by irradiation with microwave energy at 2.54GHz. The foam was dried at 100° C. and subsequently conditioned at 220°C. for 30 min. The melamine-formaldehyde foam thus prepared washydrophilic and had a density of 10 g/l. Formaldehyde emissions after 1h of storage in water at 40° C. were 150 mg of formaldehyde/kg of foam.The teabag test Free Swell Capacity (FSC) was 103 g/g. The foam had aBET specific surface area of 5.3 m²/g.

Comparative Example 2

Seventy parts of a spray-dried melamine-formaldehyde precondensate(molar ratio 1:1.6) were dissolved in 30 parts of water. This resinsolution was admixed with 3% of an emulsifier mixture of an alkanolamideand an ethoxylated fatty alcohol and also with 3% of formic acid and 10%of pentane. The mixture was foamed, and the foam dried and conditioned,as described in comparative example 1. The foam thus prepared washydrophobic and its density was likewise 10 g/l. Formaldehyde emissionswere less than 20 mg of formaldehyde/kg of foam.

Inventive Examples 1 to 15

The foam prepared according to comparative example 2 was cut into layers5 mm in thickness, which were placed in 1% aqueous solutions of thecoatings reported in table 1 and completely wetted by flexing. Each foamsample was removed from the solution after 30 min, squeezed off andpredried in air for about 18 hours. The samples. were subsequently driedat 110° C. under reduced pressure for 1 hour. Thereafter, all thetreated foam samples were hydrophilic, cf. Table 1.

Preparation of Surfactant 1

A 53% aqueous solution of a polyethyleneimine (121 g) having a numberaverage molecular weight of about 5000 was dewatered at 120° C. and 25mbar. At 80° C. the polyethyleneimine, which was stirred under nitrogen,was admixed with 15.5 g of C₁₂-C₁₄-fatty acid (acid number 271 g ofKOH/g) (EDENOR® C 12 70, from Henkel). After heating, the batch wasstirred at 160° C. for 6 hours. The water of reaction formed wasdistilled off. After cooling to 140° C., 51.8 g of formic acid wereadded dropwise and the batch was subsequently stirred at 140° C. for 4.5hours. After the amidation had ended, the batch was cooled down to 90°C. and admixed with 230 ml of completely ion-free water with stirring toprovide 332 g of a 31.5% aqueous solution of a polyethyleneimineamidated 5.0% with C₁₂-C₁₄-fatty acid and exhaustively with formic acid.

Preparation of Surfactant 2

In a four-neck flask equipped with stirrer, dropping funnel, thermometerand reflux condenser, 20.0 g (0.1 mol) of bis(aminopropyl)piperazinewere dissolved in 200 g of acetone. 22.2 g (0.1 mol) of isophoronediisocyanate were added dropwise in such a way that the temperature didnot rise above 30° C. The reaction mixture was refluxed for a furtherhour and then admixed with 110 g of HCl (1 N) and 100 g of water. Theacetone was then. distilled off under reduced pressure to leave apolyurea solution having a solids content of 16.7% by weight and a pH of7.2.

Measuring conditions of the K values for the polymers mentioned in Table1:

Polymer concentration of solution [% Polymer Solvent by weight] pH ofsolution Polyvinylamine 3% aqueous 0.5 11.0 NaCl solution Polyacrylicacid Water 1.0  7.0 (neutralized with NaOH) Polylysine Water 1.0 10.3Copolymer of 3% aqueous 0.1  8.0 acrylamide and NaCl solutionvinylimidazole Copolymer of 3% aqueous 0.1  4.8 acrylamide and N- NaClsolution trimethyl- ammoniumethyl acrylate chloride

TABLE 1 Example Coating Hydrophilic Hydrophobic Comp. 1 — X Comp. 2 — XInv. 1 Diglycerol monooleate X Inv. 2 Sorbitan monooleate X Inv. 3Reaction product of an X unsaturated C₁₃C₁₅ oxo alcohol with 4 ethyleneoxide units and 4 propylene oxide units Inv. 4 Mixture of 25% of stearylX alcohol and 75% of a reaction product of cetylstearyl alcohol with 6ethylene oxide units Inv. 5 Polyacrylic acid, K value X 110 Inv. 6Polyvinylamine, K value 90 X Inv. 7 N-Methylaminopropyltri- Xmethoxysilane Inv. 8 Amino-modified silicone- X polyether copolymer,NUWET ® 300 from OSi Inv. 9 Polyalkylene oxide- X modifiedpolydimethylsiloxane, Nuwet 500 from OSi Inv. 10 Organo-modified Xpolymethylsiloxane, NUWET ® 100 from OSi Inv. 11 Reaction mixture ofpoly- X vinylamine (K value 90) and ethylene glycol diglycidyl ether inweight ratio of 40/1 Inv. 12 Copolymer of acrylamide X andvinylimidazole having a molar ratio of 1/1, K value 30 Inv. 13 Copolymerof acrylamide X and N—trimethyl— ammoniumethyl acrylate chloride havinga molar ratio of 1/1, K value 30 14 Surfactant 1 X 15 Surfactant 2 X

Inventive Example 16

The melamine-formaldehyde resin foam hydrophilicized according toinventive example 10 was cut into layers 2 mm in thickness. Acommercially available diaper was carefully cut open, the highloftremoved and instead the 2 mm thick foam layer inserted into the diaper.The diaper was then resealed and the times taken to absorb 3 successiveadditions of 60 ml of synthetic urine were recorded The measured valuesare reported in Table 2.

Inventive Example 17

Inventive example 16 was repeated except that a foam hydrophilicizedaccording to inventive example 6 was incorporated into a diaper arid.the acquisition times were measured. The results are reported in Table2.

Comparative Example 3

A commercially available diaper was carefully cut open, the highloftremoved and then reinserted and the diaper resealed. This procedure wasintended to ensure optimum comparability. The acquisition times werethen determined. The results are reported in Table 2.

Comparative Example 4

The low-formaldehyde foam of melamine-formaldehyde condensate preparedaccording to comparative example 2 was incorporated into a diaper asdescribed in inventive example 16. The acquisition times were thendetermined. The results are reported in Table 2.

Comparative Example 5

The foam of melamine-formaldehyde condensate prepared according tocomparative example 1 was incorporated into a diaper as described ininventive example 16. The acquisition times were then determined. Theresults are reported in Table 2.

TABLE 2 Time to absorb Time to absorb Time to absorb second 60 ml third60 ml Diaper first 60 ml [sec] [sec] [sec] Comparative 7 19 29 example 3Comparative 80 125 148 example 4 Comparative 3 6 8 example 5 Inventive 34 6 Example 16 Inventive 4 6 8 Example 17

Table 2 reveals that the acquisition of the low-formaldehyde foamhydrophilicized are significantly better than those of a commerciallyavailable diaper and equivalent to those of the original,high-formaldehyde foam of comparative example 1.

What is claimed is:
 1. Hydrophilic open-celled resilient foamscomprising melamine-formaldehyde resins, characterized by a dropletabsorption rate of less than 5 seconds and an EU standard EN ISO 14184-1formaldehyde emission of less than 100 mg of formaldehyde/kg of foam. 2.The hydrophilic open-celled resilient foams as claimed in claim 1,characterized by a density of from 5 to 200 g/l, a specific surface area(determined according to BET) of more than 0.5 m²/g and a Free SwellCapacity of more than 20 g/g.
 3. The hydrophilic open-celled resilientfoams as claimed in claim 1 or 2, characterized by a tensile strengthof >60 J/m² in the wet state.
 4. A process for preparing hydrophilicopen-celled resilient foams as claimed in claim 1 which comprises (a)heating an aqueous solution or dispersion comprising amelamine-formaldehyde precondensate, an emulsifier, a blowing agent anda curing agent to form a foam and crosslink the precondensate, (b) thenconditioning the foam at from 120° C. to 300° C. for from 1to 180minutes to remove volatiles, and (c) treating the foam during theconditioning or thereafter with at least one hydrophilicizer and/or withozone, a corona discharge or a plasma.
 5. The process as claimed inclaim 4, wherein the melamine-formaldehyde precondensate has a molarratio of melamine to formaldehyde in the range from 1:1.0 to 1:1.9. 6.The process as claimed in claim 4, wherein the melamine-formaldehydeprecondensate used has a molar ratio of melamine to formaldehyde in therange from 1:1.3 to 1:1.8.
 7. The process as claimed in claim 4, whereinthe hydrophilicizer comprises at least one surfactant.
 8. The process asclaimed in claim 4, wherein the hydrophilicizer comprises a polymercontaining amino and/or ammonium groups.
 9. The process as claimed inclaim 4, wherein the hydrophilicizer comprises a polyalkylene glycol, apolymer of monoethyleneically unsaturated carboxylic acids, or a mixturethereof.
 10. The process as claimed in claim 8, wherein thehydrophilicizer comprises a polymer containing vinylamine units, apolyethyleneimine, or a mixture thereof.
 11. Hydrophilic open-celledresilient foams prepared by the process of claim
 4. 12. A method ofacquiring, distributing, and immobilizing body fluids comprisingcontacting the body fluid with a hydrophilic open-celled resilient foamof claim
 1. 13. A hygiene article to acquire, distribute, and immobilizebody fluids comprising a hydrophilic open-celled resilient foam ofclaim
 1. 14. The article of claim 13, wherein article is selected fromthe group consisting of infant diapers, incontinence products, femcarearticles, wound contact materials and secondary wound dressings.